According to Geoffrey
of Monmouth, Amphibalus was the name of the underground Christian
priest, sheltered by Britain's protomartyr, St.
Alban, at his house in the Roman town of Verulamium (now St.
Albans). The latter took his place when the authorities arrived to arrest
Amphibalus. He suffered execution for his trouble. Bede describes these
events as occurring during the religious persecutions of the Emperor
Diocletian (c.AD 304), though modern historians have argued the reigns of
Decius (c.AD 254) or Septimus Severus (c.AD 209).

Unfortunately, though Amphibalus may
have existed in person, this was almost certainly not his name. Rather,
the word is a misunderstanding of the Latin used for the cloak from which
Alban created his disguise. All other details of the man's life are, no
doubt, later medieval embellishments. He was supposedly a native of Isca (Caerleon),
converted numerous Romano-Britons after his brush with death - including SS.
Stephanus & Socrates - and fled with them to western Britannia
Superior (Wales). He was, later, caught and returned to Verulamium where
he too suffered martyrdom on 25th June.

Amphibalus' body was supposedly
discovered at Redbourne, in 1178, and translated to a fitting shrine
in the Abbey Church of St. Albans. He is, doubtfully, said to have had a
church dedicated to him in post-Roman Winchester.